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Physicists chase Einstein’s equivalence principle down a hole

By Dan Stober

Physicist Mark Kasevich works in a 25-foot pit beneath the Varian Building in search of Albert Einstein. Or more specifically, Kasevich is searching for proof that Einstein got it right in 1907 when he formulated his equivalence principle, declaring in effect that the tug of gravity is indistinguishable from the force that pushes you back into your seat in a rapidly accelerating Porsche.

If Einstein was right, the equivalence principle also requires that "objects should fall at the same rate under gravity, regardless of their composition, regardless of their mass," said Jason Hogan, one of Kasevich's graduate students. Their team is now installing the esoteric equipment designed to test that prediction by tossing up a handful of rubidium atoms—some slightly heavier than others—and watching them fall to the bottom of the pit.

Sound familiar? Galileo supposedly tried something similar, dropping objects of different weights from the Tower of Pisa. He accurately recorded that they landed together, a finding that still holds true for ordinary objects. But on the quantum atomic level, theorists have some doubts.

"We want to show that our experimental data agrees with the equivalence principle to a certain number of decimal places," Hogan said. "Our goal is 15 decimals. That would be a new world record."

If they succeed, the additional decimal points might produce interesting results. "There's some recent theory that in the area of 15 to 18 digits, the equivalence principle may no longer hold," Kasevich said. "In which case we'd be looking for a way of revising the basic laws of nature."

Physics Department chair Patricia Burchat says she's fascinated by the scene of physicists in a basement pit using quantum physics—science on the smallest scale—to examine an important underpinning of Einstein's theory of relativity, which attempts to explain the far reaches of the universe.

The "pit" is a vertical cylinder 8 feet wide and 25 feet deep, built decades ago for another experiment. Safety officials required the researchers to take "confined space" training and wear a safety harness while going up and down the ladder. "It's kind of like doing rock climbing while you're assembling the apparatus," Kasevich said.

His NASA-funded experiment involves two isotopes of rubidium, identical except for their weight: Rb-87 has two more neutrons in its core than Rb-85. Before they are dropped, the atoms must be slowed from their normal 650 mph zippiness to a pedestrian pace of a few millimeters per second—the equivalent of cooling them to within a tiny fraction of absolute zero. The task will be accomplished not with refrigeration but, counterintuitively, with lasers.

The rubidium atoms will be bombarded head-on by a stream of laser photons, slowing them to a crawl. "It's basically like trying to slow something down by throwing a bunch of snowballs at it," Hogan said.

A powerful magnetic field will confine the sluggish atoms into a millimeter-wide cloud at the bottom of the drop tube. From there, more photons will gently loft the atoms to the top of the tube to begin their freefall. Remarkably, the experimenters will be able to hit each rubidium atom with the same number of photons, about 2,000 per atom. During the atoms' 1.3-second fall back to the bottom of the tube, flashing lasers will track individual atoms as gravity accelerates their decent. Kasevich expects to loft his first atoms in about six months.

The equivalence principle was important to Einstein's understanding of space and time, according to theoretical physicist Savas Dimopoulos. Einstein realized that a person in a rapidly falling elevator would feel weightless. If she were to release a ball from her hand, it would not fall to the floor, but would hover next to her. From her perspective, the effects of gravity would be cancelled.

"So the equivalence principle says you can undo the effects of gravity by going with the flow," Dimopoulos said. If that's the case, "then perhaps gravity is a property of space; it has nothing to do with the objects themselves, their masses, their color, the constitution of the object."

The equivalence principle led Einstein to suggest that objects move toward each other not as a result of mutual gravitational attraction, as Newton believed, but because the objects change the shape of space itself, in effect causing them to roll downhill toward each other.

If the Stanford experiment proves Einstein wrong—if the heavier rubidium atom falls faster than the lighter one—it could mean that there's another undiscovered particle afoot in the universe, Dimopoulos said. It would be less democratic than gravity—it would treat one atom differently from another, as do electricity and magnetism.

"The most exciting time in the life of a theorist is when one of these wild predictions turns out to be true," he said.